1. Field of the Invention
This invention relates to thermionic filaments. In particular, this invention relates to apparatus and methods for sustaining thermionic emission in mass spectrometry systems.
2. Background Information
Mass spectrometry is a technique for analyzing a specimen, which may include a solvent in addition to a sample containing one or more analytes. Mass spectrometric analysis is based on the dependence of ion trajectories through electric and magnetic fields on respective ion mass/charge ratio. The prevalence of constituent ions in the specimen is measured as a function of mass/charge ratio and the data are assembled to generate a mass spectrum of the specimen. In a common operational mode, known as electron ionization, the specimen is ionized by means of bombardment by thermionically emitted electrons in an ion source.
The thermionic electron source typically incorporates a filament of a high-melting metal such as tungsten, rhenium, iridium, platinum or an alloy thereof. The emission current of thermionically emitted electrons is related to, among other parameters, the surface area of the filament. Consequently, a filament morphology incorporating bends or turns, such as a coil, may augment the emission current from a given instrumentation volume, compared to a straight wire filament.
Analysis by mass spectrometry is commonly enhanced by combination with analytical techniques that separate the specimen into constituents before ionization in the mass spectrograph. For example, in a common enhancement a gas chromatograph separates the specimen into constituents before it arrives at the spectrometer ion source. This arrangement, termed gas chromatography-mass spectrometry (“GC/MS”), is widely used to identify unknown samples, for example in environmental analysis and drug, fire and explosives investigations. The separative powers of gas chromatography enable GC/MS systems to identify substances to a much greater certainty than is possible using a mass spectrometry assembly alone.
Elution from the gas chromatography unit of the solvent portion of the specimen may stress the thermionic electron source due to its relatively large volume and concomitant quantity of ionizable entities. Consequently, the thermionic filament in a GC/MS system is typically kept unenergized as the solvent fraction passes through the ion source, during a time period known as the “solvent delay.” Power to the thermionic filament is cycled on when the specimen constituents dissolved in the solvent are eluting. Solvent delay is known to protect the ion source thermionic filament from one type of lifetime-limiting stress. However, cycling of the filament current brings its own hazards to the filament. Alternating on and off phases cycles may give rise to stresses on the coiled filament due to thermal expansion and electromagnetically induced effects. As a result, relative movement between portions of a coiled filament, such as adjacent turns, may bring the portions into contact, ending the usefulness of the filament. In addition, a coiled filament may creep after cycling and prolonged high-temperature operation until the filament is no longer optimally placed within the ion source, compromising its efficiency. Increasing the filament current to reestablish the desired emission current may exacerbate degradation of the coil shape rather than improving performance.
There is, accordingly, a need for a GC/MS ion source thermionic filament that is robust to time variations in the filament current and long-term creep.